Intranuclear inclusions characteristic of parvovirus B19 infection were detected in the ten fetal tissues of hydrops fetalis. Cells containing nuclear inclusions showed characteristic figures of apoptosis. With the TUNEL immunostaining the erythroid cells showed intense signals. An erythropoietin-dependent strain of a megakaryoblastic cell line was inoculated with B19 virus, which were positive for B19 viral capsid protein. The nucleus of the infected cells showed the characteristic figures of apoptosis and the positive staining with TUNEL. DNA fragmentation analysis by gel electrophoresis showed the characteristic DNA fragmentation for the cells infected in vitro. These results indicate that B19 infection in vitro induces apoptosis of infected cells. Erythroid lineage cell lines with the stringent expression of NS1 were established. The initiation of cell death occurred after the induction of the NS1 protein. The death cells showed apoptotic appearance and characteristic DNA fragmentation on gel electrophoresis. Using the TUNEL method, we detected cells positively stained for fragmented DNA. NS1 contains an NTP-binding motif and the cytotoxicity mediated by NS1 is abolished by various mutations within the NTP-binding domain. We genetically engineered the mutants with disruption in the NTP-binding domain, which dramatically suppressed the cytotoxic activity of NS1. These results suggest that NS1 directly mediates the apoptosis of these erythroid cell lines. The fetal tissues were stained with antibodies against cellular factors in the signaling pathway of apoptosis. Intense signals were observed with the antibody to caspase3 in erythroid cells in the tissues. No positive signals were detected with antibodies to caspase1, caspase2, FAS, Bcl-2, perforin, or gramzyme B. The results indicate caspase3 plays an important role for the induction of apoptosis to the erythroid cells by B19.